Although the underground exploration industry has made technical progress over the last decades, it is still impossible to predict with high certainty the outcome of a planned geological exploration campaign. Today due to the poor underground mapping there is still a considerable element of luck in mining exploration or high risk that a cavity or any other soft ground layers will not be mapped in civil constructions projects. The existing underground mapping techniques suffer from limited survey distance due to the high attenuation of the ground.
In mineral exploration, multiple measurements and tools are used and combined: (a) geophysical exploration: seismic reflection and refraction, gravity survey, magnetic survey using proton magnetometer, electrical resistivity and downhole logging survey; (b) geochemical exploration methods: soil sampling and stream sampling; and (c) direct exploration methods: Drilling and Mapping.
Down-hole logging surveys are made using a wide range on instruments that can be lowered into a borehole to gather information about the borehole itself and about the physical and chemical properties of rock, sediment, and fluids in and near the borehole. The down-hole logging instruments can be classified by the type measurements they perform: (a) Mechanical methods that include caliper logging and sonic logging; (b) Electrical methods that include resistivity and conductivity logging, spontaneous potential logging and measurements of induced polarization, and (c) Radioactive methods that include Natural gamma ray logging and Porosity logging using neutrons from a radioactive source
The main limitation of the existing down-hole logging surveys is their limited survey distance from the borehole. There is a need to reduce the number of the drilling, and to collect as much as possible data from existing drillings in order to determine about the next steps of the drillings.
Cosmic ray muons are part of the naturally occurring cosmic radiation. Cosmic rays muons are the most-penetrating charged particles on earth. The muons arrive at the earth's surface with energies ranging from less than a GeV to thousands of GeVs. The flux of cosmic ray muons at the earth's surface is from 100 to 200 per square meter per second, depending mostly on the minimal energy considered, but also on the latitude, the weather, and other less significant variables. At the high energies relevant to underground mapping, the muons' initial directions are fairly isotropic, while at lower energies the muons tend to move towards the nadir.
Muons lose energy as they travel through matter. For muons with energies below 200 GeV, energy is lost mainly through ionization. Underground, cosmic-ray muons typically decay after reaching non-relativistic energies, for example below 0.2 GeV.
The muon energy loss is proportional to the mass of the matter the muon traversed. Due to the chemical composition of mantle rocks, the effects of the additional dependence on the chemical composition of the transversed material are negligible. The denser rocks result in larger energy loss and in fewer muons that penetrate through these rocks. Thus a map of the rates of muons arriving at an underground sensor provides a map of the weight above the sensor. This basic correspondence has been used successfully in archeology and in mapping volcanos.
The first to map rocks with muons was probably Alvarez, who searched for hidden chambers in the Egyptian pyramids. Alvarez simply divided the 2D angular phase space into discrete regions (known as “bins” in this context) and compared the observed and expected muon counts in each bin.
There is a growing need to provide reliable mapping methods.
Gaseous Detectors have shown remarkable performance in accelerator based experiments and in research labs.
These detectors require ongoing maintenance (e.g. gas flow)—and thus prior art gaseous detectors cannot and are not used on commercial and out of lab environment. Several detector technologies can be used to detect ionizing particles and measure their entry, such as scintillating fibers, ring-imaging Cherenkov detectors, and various gas detectors.
When a muon passes through a gas, it typically ionizes a few molecules per millimeter. The ionization rates roughly scale with the mass density, and so are far higher in liquids and solids.
In a gas detector, high voltage is applies across the gas so that the electrons from an ionization event are amplified in the gas, first forming an electron avalanche, then possibly a streamer, then possibly a spark. Different gas detector technologies differ in how far they allow the amplification process to go (e.g. are sparks desired or avoided), in the mechanisms used to prevent transverse growth of the shower, in the geometries of the regions with high electric fields where amplification takes place, in their preferred gas mixtures, which depend on the choices listed earlier, in the arrangement of the readout, in the removal of charges from previous signals, etc.
A detector of the Micro-Pattern Gas Detector (MPGD) family can achieve efficient and stable gas amplification, by concentrating the electrical potential difference in a small volume. In the Gaseous Electron Multiplier (GEM) family, these small volumes are holes in the GEM layer, which in the original GEN design is a film. Various gas mixtures are used in GEM detectors, typically with 70%-95% of the mixture a noble gas and one or more quencher gases such as CH4, N-pentane, CO2, or Dimethyl Ether.
In Thick-GEM (ThGEM) detectors, the GEM layer is a Printed Circuit Board (PCB). The GEM layers in ThGEMs can be produced using existing large-scale commercial Printed Circuit Board (PCB) production techniques. In particular, the holes are drilled, and in what followed we refer to this layer as the Drilled Board (DB).
Generic MPGDs suffer from sparks that can harm the MPGDs and the readout electronics. Sparks can also cause chemical reactions in the gas, particularly if it contains hydrocarbons, such as N-pentane, which is commonly used as a quencher gas. GEM detectors typically employ two or three amplification layers, while ThGEM can offer higher amplification per layer are typically constructed with one to three amplification layers.
The small signals (104-106 electrons) collected on the readout board must be amplified electronically. The amplifiers are best located close to the readout board, to minimize interference and capacitance on the lines that carry the small analog signals. Typically they are integrated within front-end electronics (FEE) that also digitize the signals and provide trigger information.
Some of the detectors in the ThGEM family have been designed to avoid sparks. In particular, we note the Resistive-Plate Well detectors (RPWell) and Resistive Anode Well (RWell) detectors.
In an RPWell, a plate with large volume resistivity is placed between the DB and the readouts, and signal charges are evacuated through this plate to the readouts. In an RWell, a thin coating or film with high surface resistivity is placed between the DB and the readouts, and a thin insulator (a sheet of FR4) is placed between the conductive layer and the readouts. The edges of the RWell's resistive layer are grounded, so that the signal charges are evacuated along the resistive layer to the sides. Both RPWell and RWell detectors employ a “well” geometry, where the last amplification layer is adjacent to the anode, without a gas gap between them.
Gas amplification detectors rely on gas flow in the detector to remove trace contaminations from the gas, especially electronegative gasses such as water vapor, Flour, Chlorine, and complex molecules such as hydrocarbons and halogenated hydrocarbons. Such contaminations can arise from internal leaks, from environmental materials permeating through the seals, and from outgassing from the detector components. The latter includes any electronics and wiring within the gas volume. Gas contaminations can degrade detector performance through two main mechanisms. First, such contaminants can capture electrons in the gas. In the drift gap they can capture ionization electrons before they reach the DB, and in the DB they can reduce the effective gain, in either case, reducing the detector's efficiency at any operating voltage. The second mechanism is through the electron avalanches, which can induce chemical reactions in some contaminants. For example, hydrocarbons might polymerize, releasing soot which can settle on the DB and result in sharp conductive edges on its electrode. Such edges might reduce the maximal voltage maintained in the DB below the minimal operating voltage. The sensitivity of the detector to the different chemicals varies by over an order of magnitude, so concepts such as the “total contaminant concentration” are of little use, and when such numbers are quoted here, they should be taken as indications of possible values.
There is a need to provide improved detectors.